The advantages of using impulse or short pulse radar for detecting discontinuities in dielectric media is well recognized as disclosed in U.S. Pat. No. 4,072,942 to Alongi, U.S. Pat. No. 4,008,469 to Chapman and U.S. Pat. No. 3,806,795 to Morey. Systems of the type disclosed in these patents operate by radiating a pulse of only one or a few excursions containing a broad spectrum of radio frequencies enabling the system to detect target phenomena of widely varying characteristics. When radiated into a dielectric medium, short pulses are reflected by discontinuities in the medium in a manner which allows the pulse echo to be detected and analyzed to provide information about the location and size of the discontinuity. Generation of a short pulse in real time, however, involves two serious drawbacks. The first is the necessity of recording the high-frequency data with a reasonable dynamic range, while the second is the problem of designing an antenna capable of coupling the broad band energy efficiently into the ground. The first problem can be solved in part by using a hetrodyne receiver technique. The problem of efficient antenna coupling is much more difficult. Normally, attempts to solve this problem have involved building a broad band antenna designed to have a minimum of reflections. While such an antenna operates well to transmit energy into the medium, it is by necessity a low-gain antenna system.
One prior attempt to overcome the drawbacks of real time pulse radar has been the development of a synethetic short pulse radar in which continuous wave measurements are made at many selected frequencies defining a Fourier spectrum of frequencies equivalent to the bandwidth of a short radio frequency pulse. This prior art system is disclosed in Robinson, L. A. et al., "Location and Recognition of Discontinuities in Dielectric Media Using Synthetic R F Pulses," Proceedings of the IEEE, Vol. 62, No. 1, January 1974, pages 36-44 and in Robinson, L. A. et al., "An R F Time-Domain Reflectometer Not in Real Time", IEEE Transactions on Microwave Theory and Tech., Vol. MTT-20, pages 855-857. In the system disclosed in these articles, a computer is used to control the sequence of measurements, to store the measured parameters and to process the stored parameters to permit a synthetic pulse echo to be displayed. Since the amplitudes and phases of the spectral lines can be individually controlled, the synthetic radar pulse may be shaped to achieve optimum tradeoff between short pulse width, small ringing on the baseline between pulses, and total bandwidth covered by the spectrum.
While synthetic radar pulse systems possess numerous theoretical advantages over real time pulse radar, the circuitry designs employed heretofor to implement such synthetic pulse systems have involved considerable impractical complexities. For example, the circuitry disclosed in the above noted Robinson et al. articles include a "tunable" oscillator requiring external frequency-stabilizing circuitry to obtain the necessary degree of stability in the frequency output of the oscillator. Such frequency-stabilizing circuitry adds to the complexity and cost of implementing such a system. Moreover, a "tunable" oscillator would not be directly compatible with standard digital logic circuitry such as a microprocessor system which, given the present state of the art in circuit design would provide the least expensive and most reliable approach toward implementation of the control circuitry of a synthetic pulse system. Moreover, no prior art synthetic short pulse radar system has disclosed a short pulse frequency spectrum which has been shown to be ideally suited for detection of particular targets such as hazards within a coal mine.
Other forms of known subsurface target detecting systems employing radar are disclosed in U.S. Pat. Nos. 3,903,520; 3,831,173 and 3,903,520 but none of these systems embodies the advantages of synthetic R F pulse radar wherein the transmitter and receiver circuit designs are compatible with integrated circuit control components.